FDS is a fundamental part of the file system in APFS formatted drives. It is not something users need to enable/disable, and it's not something that can apply to one directory and not another.
As it's an 'all in' scenario, there aren't any terminal commands to determine if a directory has FDS enabled. Instead, if a drive is formatted in the APFS format then FDS is enabled. If it's not formatted in the APFS format, then FDS isn't available.
Unfortunately, the information you've quoted from Apple's technical documentation is badly worded, hence the reason for your question.
If a drive is formatted in the APFS format then Fast Directory Sizing is applied across the board. FDS is not something a user can enable/disable on a per directory basis.
Because of this, it's also not something a user can determine the status of on a directory by directory basis. That is, if the drive is formatted as APFS then all directories have FDS enabled.
Where the confusion comes from (and understandably so in this case) is the bad choice of words in Apple's documentation and the ambiguity that created. What this information was trying to convey is that a user will benefit most from FDS in those directories that have lots of files but change little (e.g. a user's Documents folder), whereas they will not get much benefit (if any) from FDS in directories like the
As an aside, the documentation you referenced has since been replaced by the About Apple File System page. The only mention of FDS on this page is as follows:
Apple File System offers improved file system fundamentals as well as several new features, including cloning, snapshots, space sharing, fast directory sizing, atomic safe-save, and sparse files.
There is no attempt to further explain what FDS means, beyond the fact it's a fundamental (read underlying) feature. However, if you're interested in some background:
In complex and deep directory structures in HFS+, we are familiar with the time the ﬁle system could spend on calculating the size of the ﬁle system tree. This is one of the common reasons that the system would hang. Apple, with APFS, introduced Fast Directory Sizing (FDS), where the ﬁle system can quickly compute the space used by a directory hierarchy, in order to remove this problem.
Source: Hansen, K.H., Toolan, F., Decoding the APFS ﬁle system, Digital Investigation (2017)